When providing a connection in a communication network, usually several network elements are involved, including the connection originating network element, the connection terminating network element and/or one or more intermediate network elements such as a base station, a base transceiver station, a base station controller and/or one or more support nodes handling the signalling and/or user traffic.
As an example, in a GPRS-based or UMTS-based network, a connection (e.g. call) originating from, or terminating at, a user equipment such as a mobile station (MS) is made to a connection terminating or originating equipment using a radio network controller (RNC) which communicates with a SGSN (Serving GPRS Support Node) and possibly a GGSN (Gateway GPRS Support Node). The connection terminating or originating equipment can be located in the same or a different network. In particular, in case of mobile user equipments, the actual location thereof network. In particular, in case of mobile user equipments, the actual location thereof is defined with a resolution of a routing area (e.g. in idle state) or with a finer resolution of a cell (e.g. when handling a connection such as a call). Note that Routing area (RA) is a standard term used in conjunction with GPRS, while GSM and UMTS Circuit Switched systems use the term Location Area (LA). In both case, the area is referring to the area where a mobile station is registered in the serving node (e.g. SGSN or MSC/VLR), and where eventually the serving node pages the mobile station to establish downlink connection. In this application, the term area will be used to refer to location area and/or routing area.
The coverage area of an entire network is usually divided in several areas (RA or LA), with one area (in a GPRS- or UMTS-based network) being assigned to one serving node (one serving node typically handling many areas). When having information on the area where the user equipment is presently located, the serving node in charge of handling a connection to or from this user equipment is unambiguously defined.
For example, in GSM and UMTS, this one to one correlation between the routing or location areas and the assigned SGSNs may, however, be of disadvantage in case of break-down of an SGSN or necessary maintenance operations such as software updating. In such a case, the routing area has to be completely shut-down and is at least temporarily no longer usable for providing connections.
This situation may be significantly improved when changing the network structure in such a manner that at least two serving nodes such as two SGSNs are able to handle the same routing area. In such a case, e.g. a base station controller (BSC) or radio network controller (RNC) may use different interfaces such as Iu and/or Gb. Several types of mobile stations may be supported by using two radio interfaces and providing only one single base transceiver station (BTS).
The provision of two or more support nodes serving the same routing area provides several advantages such as resilience by enabling an RNC (possibly having a list of available SGSNs) to use another SGSN if the previously used SGSN should become overloaded or out of order. Furthermore, maintenance operations such as software updates can be effected without shutting down the area. In addition, the network signalling caused by inter-SGSN hand-over can be reduced.
As an example, several SGSNs may be provided for covering a metropolitan area such as London area, and a mobile station moving around the city can always use its original SGSN for handling connections.
For instance, an IP network may be introduced on an interface such as Iu interface which presently is mainly used as a point-to-point Iu interface between the RNC and the SGSN. When introducing an IP network or network of some other appropriate type on the Iu interface, one RNC may be connected to several SGSNs.
In a case where one network element (which e.g. is in charge of controlling the radio connection to a user equipment) is able to connect to different support nodes being alternatively provided, there exists a problem in finding and selecting an appropriate support node, for instance an SGSN to be used for a signalling connection. This signalling connection may e.g. be used to transfer L3 (layer 3) messages (such as mobility management MM and session management SM) between the user equipment (e.g. MS) and the support nodes such as SGSN. Furthermore, in case of inter-support node hand-over the new support node would benefit from finding the old support node which was serving the user equipment until hand-over.